Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Scaptĭa, ae, f., a very ancient town in Latium, now Passerano, Plin. 3, 5, 9, § 68.
Hence, Scaptĭus, a, um, adj., of Scaptia, Scaptian: tribus, Liv. 8, 17 fin.
Hence, Scaptĭensis, e, adj., of the Scaptian tribe: tribules, Suet. Aug. 40.